Climatic characteristic and changes of snow cover over the Northern Hemisphere,Eurasian,North America and the Tibet Plateau are studied based on data of the Northern Hemisphere snow cover from 1966 to 2000.Correlation analysis shows that:(1) a inverse relationship between winter snow cover over Eurasia and monsoon rainfall over India,and Indian monsoon rainfall is not only influenced by snow cover over Eurasia,but also by ENSO.(2) there is an obvious relation between snow cover over the Tibet plateau during October to November and the following onset and rainfall of Indian monsoon,and the possible effective mechanism is proposed.